1. Technical Field
The present invention relates to a method of fabricating a thin-film transistor, a thin-film transistor, a display device, a sensor, and an X-ray digital imaging device.
2. Related Art
The research and development of thin-film transistors that use an In—Ga—Zn—O (hereinafter called IGZO) oxide semiconductor thin film at the active layer (channel layer) has been flourishing in recent years. Oxide semiconductor thin films can be formed at low temperatures, and exhibit higher mobility than amorphous silicon, and further, are transparent with respect to visible light. Therefore, a flexible thin-film transistor can be formed on a substrate such as a plastic plate or a film or the like.
Here, a comparative table of the field effect mobilities, process temperatures and the like, that are characteristics of respective types of transistors, is shown as Table 1.
TABLE 1LTPSlowtemperatureOrganic-Oxide-polysilicona-Si:HμC—Si:HTFTTFTmobility100<12~3<1~53~50stability   VTH<11001~2301~2 uniformityΔ◯◯?Δ◯?film formation450300300RT~100RT~350temperature
As shown in Table 1, at a thin-film transistor whose active layer is polysilicon, a mobility of around 100 cm2/Vs can be obtained, but the process temperature is greater than or equal to 450° C. which is extremely high. Therefore, a thin-film transistor whose active layer is polysilicon can only be formed on substrates having high heat resistance, and is not suited to low costs, large surface areas, and making structures flexible. Further, because a thin-film transistor whose active layer is amorphous silicon can be formed at a relatively low temperature of around 300° C., the selectivity of substrates is broad as compared with polysilicon, but only a mobility of around 1 cm2/Vs at best can be obtained, and a thin-film transistor whose active layer is amorphous silicon is not suited to high-definition display applications. On the other hand, from the standpoint of low-temperature film formation, thin-film transistors whose active layer is an organic substance can be formed at less than or equal to 100° C., and therefore, application to flexible display applications using a plastic film substrate or the like that has low heat resistance, or the like, is anticipated, but with regard to the mobility, only effects that are of the same extent as those of amorphous silicon can be obtained.
In the fabrication process, carrying out a heat treatment (post-annealing processing) after element formation in order to improve the electrical characteristics of the transistor is often seen. At a thin-film transistor using an oxide semiconductor thin film at the active layer, a device exhibiting high mobility and high reliability can be formed even by low-temperature formation as described above, and lowering of the temperature at the time of the heat treatment is anticipated.
In a thin-film transistor using an IGZO oxide semiconductor thin film at the active layer, several methods for controlling to the desired electrical characteristics by a heat treatment have been reported.
Japanese Patent Application Laid-Open No. 2008-53356 discloses, in a thin-film transistor whose active layer is IGZO, carrying out a heat treatment in a temperature range of 250° C. to 450° C. after active layer formation. Further, carrying out a heat treatment in a temperature range of 150° C. to 450° C. while irradiating oxygen radicals and ozone is also disclosed.
Japanese Patent Application Laid-Open No. 2010-238770 discloses, in a thin-film transistor whose active layer is IGZO, carrying out a heat treatment in a temperature range of 200° C. to 500° C. after active layer formation. Further, as an example of this method, in Example 3 of Japanese Patent Application Laid-Open No. 2010-238770, there is disclosed film forming an active layer that is formed from IGZO in a mixed gas atmosphere of oxygen and argon having an oxygen concentration of 1%, and thereafter, carrying out a heat treatment at 300° C. in an atmosphere of 100%.
However, in Japanese Patent Application Laid-Open No. 2008-53356 and Japanese Patent Application Laid-Open No. 2010-238770, when carrying out a heat treatment at a high temperature of 300° C. or more, there is the problem that, from the standpoint of heat resistance, it is difficult to use such methods for transistors that are formed on flexible substrates such as plastic substrates or the like. Further, when carrying out a heat treatment at a low temperature of less than 300° C., lowering of the resistance may occur, and therefore, it is difficult to obtain a normally-off transistor merely by simply adjusting the heat treatment temperature. Moreover, when carrying out a heat treatment at a low temperature of less than 300° C., the electric field mobility is, for example, around 7.1 cm2/Vs (refer to Experimental Example 3 of Japanese Patent Application Laid-Open No. 2010-238770), and is not greater than or equal to 10 cm2/Vs. Still further, when carrying out a heat treatment while irradiating oxygen radicals and ozone as in Japanese Patent Application Laid-Open No. 2008-53356, high stability at a temperature range of 150° C. to 450° C. is obtained, but the process becomes complex.